Method of fabricating a MOS device

ABSTRACT

Deposited dielectric layers for a semiconductor device are typically formed in a chemical vapor deposition. Often a hydrogen by-product is formed. Especially in a plasma enhanced chemical vapor deposition process, the hydrogen by-product can form free radicals that are introduced into the dielectric layers. The hydrogen free radicals can affect the stability of the threshold and breakdown voltage of MOSFET transistors. Deuterium introduced into the CVD chamber competes to enter the dielectric layer with the hydrogen. The deuterium prevents some of the hydrogen free radicals from entering the dielectric layer and thus increases MOSFET reliability.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to methods for formingsemiconductor devices. In particular, the present invention relates tomethods for forming dielectric layers on semiconductor devices.

[0002] Dielectric layers on semiconductor devices are used to isolate orprotect other layers. Currently silicon dioxide, silicon nitride andsilicon oxynitride are the most commonly used dielectric layers.

[0003] One type of dielectric layer is a thermally-grown silicondioxide. Thermally-grown oxides are formed by reacting silicon on thesubstrate with oxygen at high temperatures. Field and gate oxide layersare examples of thermally-grown oxides.

[0004] Another type of dielectric layer is a deposited dielectric layer.Deposited dielectric layers include interlevel dielectric layers such asboro-phosphorous silicate glass (BPSG), inter-metallic dielectric layersand passivation layers. Typically, deposited dielectrics are formed witha chemical vapor deposition (CVD) system. Common CVD systems includeplasma enhanced chemical vapor deposition (PECVD) and low pressurechemical vapor deposition (LPCVD). In chemical vapor deposition,reactants in a gas or vapor react to form a dielectric material which isdeposited on the substrate. To form a deposited silicon dioxide layer,typically a mixture of nitrous oxide (N₂O), silane (SiH₄) andtetra-ethyl-ortho-silicate (TEOS) are used. To form a nitride layer,such as Si₃N₄, the reactant substances include silane and ammonia NH₃.To form a combined oxynitride, a mixture of silane, nitrous oxide andammonia is used.

[0005] A concern with metal-oxide-silicon field-effect-transistors(MOSFETs) is the need to maintain stable and reliable threshold andbreakdown voltage values. It is desired to improve the processing stepsused to form the dielectric layers to help obtain stable and reliablethreshold and breakdown voltage values for MOSFETs.

SUMMARY OF THE INVENTION

[0006] The CVD deposition of dielectric layers typically uses reactantsubstances which include hydrogen. These reactant substances includesilane, ammonia and TEOS. Such CVD depositions produce hydrogen gas as aby-product. Especially when a plasma is used, the hydrogen gas can breakdown into free radicals which do not easily recombine. These hydrogenfree radicals become a part of the deposited dielectric layer as itforms.

[0007] It has been found that hydrogen can be between five to fifteenpercent by weight of a deposited dielectric layer. Because the hydrogenfree radicals are charged, hydrogen free radicals entrapped in thedielectric layer can adversely affect MOSFET performance. The entrappedcharged particles can vary the threshold voltage and breakdown voltagefor a MOSFET. Such changes may not be immediately apparent and may occurafter the integrated circuit (IC) is sent to a customer. Additionally, asignal pattern can cause the entrapped hydrogen free radicals to migrateand adversely affect the performance.

[0008] The present invention involves supplying deuterium into the CVDchamber. Deuterium competes with the more common isotope of hydrogen toenter the dielectric layer. Additionally, deuterium is more stable thanthe common isotope of hydrogen and is thus less likely to form freeradicals which can adversely affect the MOSFET performance.

[0009] Deuterium has been used in the past in the forming of polysiliconlayers. Deuterium affects the microcrystalline structure of thepolysilicon layer as it forms. See, for example, T. Shiraiwa et al.;“Characterization of Chemical-Vapor-Deposited Amorphous-Silicon Films”;Jpn. J. Appl. Phys. 32; Jan. 15, 1993; Pt. 2, No. 1A/B; pp. L20-23; andE. Srinivasan and G. N. Parsons; “Hydrogen elimination and phasetransitions in pulsed-gas plasma deposition of amorphous andmicrocrystalline silicon”; J. Appl. Phys.; Vol. 81, No. 6; Mar. 15,1997; pp. 2847-2855. In its prior use, deuterium was not used to form adielectric layer. Additionally, dielectric layers do not require aspecific micro-crystalline structure. Furthermore, in the prior art,deuterium was not used to compete with a hydrogen by-product during alayer deposition.

[0010] In one embodiment of the present invention, deuterium is added inthe deposition of the dielectric layer when the deposition is caused byreactive substances which produce hydrogen by-products.

[0011] Another embodiment of the present invention is a chemical vapordeposition system arranged with a deuterium supply and reactantsubstance supplies, where the reactant substances form a hydrogenby-product.

[0012] Another embodiment of the present invention comprises asemiconductor device having a dielectric layer including at least somedeuterium. Having a dielectric layer including at least some deuteriumhas advantages over dielectric layers that include hydrogen by-productswhich are not deuterium.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] The above and other features and aspects of the present inventionwill become more apparent upon reading the following detaileddescription in conjunction with the accompanying drawings, in which:

[0014]FIG. 1 is a diagram of a plasma-enhanced chemical vapor depositionsystem using deuterium as a supplied gas;

[0015]FIG. 2 is a flowchart of the method of the present invention; and

[0016]FIG. 3 is a diagram of a semiconductor device showing a dielectriclayer formed so as to include at least some deuterium.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0017]FIG. 1 is a diagram of a plasma enhanced chemical vapor depositionsystem 10 using deuterium 14 as a supply gas. The system shown is aplasma enhanced chemical vapor deposition (PECVD) system, but thepresent invention is applicable to other chemical vapor depositionsystems such as a low pressure chemical vapor deposition (LPCVD) system.The vapor and gas supply 12 includes the reactant supplies 16. Thereactant substances 16 in the shown embodiment are used to form asilicon dioxide layer. The reactants include silane, nitrous oxide andTEOS. The reactants can be provided as a vapor from a liquid supply oras a gas.

[0018] The reactant substances are sent through a shower head 18 into achamber 20. Since the reactant substances form hydrogen as a by-product,the formed plasma 22 will include hydrogen free radicals.

[0019] In the present invention, deuterium 14 is added to the CVDchamber 20 during at least a portion of the time in which the dielectriclayer is formed. Deuterium is more stable than the more common isotopeof hydrogen upon completion of the film deposition, and forms fewer freeradicals after the dielectric layer is deposited on the wafer 24.

[0020] An RF power supply 26 is used to produce the plasma. In apreferred embodiment, dual RF signals including 40 Hz and 13.56 MHz areused in order to reduce film stress.

[0021] In a manner known in the art, the temperature of the wafer ismaintained relatively low, preferably at about 250° C. to 420° C. Thisis especially important when the dielectric layer is formed after ametal layer, because higher temperatures can melt the metal layer.

[0022] The pump 20 a removes molecules from the chamber to produce lowpressure in chamber 20 so as to form the plasma. The removed moleculeswill include some hydrogen prevented from entering the dielectric layerby the introduced deuterium.

[0023] In a preferred embodiment, the chemical vapor deposition systemmay be a Novellus Concept One available from the Novellus Company of SanJose, Calif.

[0024]FIG. 2 illustrates a method of the present invention. In step 30,a semiconductor wafer is provided. In step 32, the wafer is placedwithin a CVD chamber. In step 34, reactant substances including hydrogenare introduced into the CVD chamber.

[0025] In step 36, the reactant substances react together to form adielectric material which is deposited on the substrate. The reactionforms hydrogen as a by-product. During at least a portion of thedielectric layer deposition, deuterium gas is supplied into the chamberso that the deuterium enters the dielectric layer. In one embodiment,deuterium is present for only part of the dielectric layer forming step.For example, deuterium can be present for the beginning, middle or endof the dielectric layer deposition. Supplying deuterium during only aportion of the dielectric layer deposition may be beneficial because thespecific composition of the dielectric layer needs to be compatible withany neighboring layer. Furthermore, the effect of free radicals from thedeposited layer on the threshold voltage and breakdown voltage of aMOSFET can be minimized through the use of deuterium in the film'sdeposition.

[0026]FIG. 3 is a diagram of a semiconductor device 50 showingdielectric layers 60, 64 and 68 including deuterium. The semiconductordevice 50 includes a silicon substrate 52. Field oxide layers 54 andgate oxide layer 56 are thermally grown on the substrate 52. Apolysilicon layer 58 is formed on top of the gate oxide 56. Aninterlevel oxide 60, such as BPSG, is formed by a deposition process.Next, a metal interconnect layer 62 is formed on top of the BPSG 60. Anintermetallic dielectric 64 is deposited on top of the metal layer 62.Layers formed after the metal layer 62 should be done at a relativelylow temperature, for example below 420° C., so as to avoid reflowing themetal layer 62. Next, the second metal layer 66 is formed over theintermetallic dielectric layer 64. Finally, a deposited passivationlayer 68 is formed on top of the metal layer 66 to protect the othersemiconductor layers.

[0027] The interlevel dielectric 60, intermetallic layer 64, andpassivation layer 68 can include some deuterium for the reasonsdescribed above. There is no reason to add deuterium to the field oxidelayer 54 and gate oxide layer 56, because hydrogen does not remain inthe film due to the temperature in the process chamber when these layersare formed.

[0028] In a preferred embodiment, over one percent by weight of adeposited dielectric layer is deuterium. Additionally, in anotherpreferred embodiment the percent by weight of deuterium is at least afifth of the percent by weight of hydrogen present in the dielectriclayer. Replacing at least fifty percent of the hydrogen with deuteriumis currently believed to be possible with the method of the presentinvention.

[0029] Various details of the implementation and method are merelyillustrative of the invention. It will be understood that variouschanges in such detail are within the scope of the invention, which isto be limited only by the following claims.

What is claimed is:
 1. A method comprising: providing a semiconductorwafer; and forming a dielectric layer on the semiconductor wafer in achemical vapor deposition process, the chemical vapor deposition processincluding introducing at least one reactant substance that includeshydrogen, the chemical vapor deposition process including the step ofintroducing deuterium during at least a portion of the chemical waferdeposition process so that at least some deuterium enters into thedielectric layer.
 2. The method of claim 1, wherein the dielectric layerforming step comprises introducing silane as the at least one reactantsubstance including hydrogen.
 3. The method of claim 1, wherein thedielectric layer forming step comprises introducing ammonia as the atleast one reactant substance including hydrogen.
 4. The method of claim1, wherein the dielectric layer forming step comprises introducingtetra-ethyl-ortho-silicate (TEOS) as the at least one reactant substanceincluding hydrogen.
 5. The method of claim 1, wherein the dielectriclayer forming step comprises introducing deuterium as a gas.
 6. Themethod of claim 1, wherein the dielectric layer forming step is done ina plasma enhanced chemical vapor deposition process.
 7. The method ofclaim 1, wherein the dielectric layer forming step is done after forminga metal layer on the semiconductor substrate, the dielectric layerforming step being done at a temperature to minimize the melting of themetal layer.
 8. The method of claim 1, wherein the dielectric layerforming step is such that hydrogen is formed as a by-product of thereaction of the reactant substances.
 9. The method of claim 8, whereinat least some of the by-product hydrogen is in the form of a freeradical of hydrogen.
 10. The method of claim 1, wherein the dielectriclayer forming step is such that the dielectric layer comprises an oxide.11. The method of claim 1, wherein the dielectric layer forming step issuch that the dielectric layer comprises a nitride.
 12. The method ofclaim 1, wherein the dielectric layer forming step is such that thedielectric layer comprises an oxynitride.
 13. The method of claim 1,wherein the dielectric layer forming step is such that the dielectriclayer is formed on a metal layer.
 14. The method of claim 1, wherein thedielectric layer forming step includes introducing deuterium at thebeginning of the process.
 15. The method of claim 1, wherein thedielectric layer forming step includes introducing deuterium at the endof the process.
 16. A chemical vapor deposition system adapted forforming a dielectric layer on a substrate, the system comprising: achamber; a supply of reactant substances, the supply adapted tointroduce the reactant substances into the chamber, the reactantsubstances being such that they form a dielectric layer, wherein atleast one of the reactant substances includes hydrogen; and a supply ofdeuterium gas adapted to introduce the deuterium gas into the chamberduring at least a portion of forming of the dielectric layer.
 17. Thechemical vapor deposition system of claim 16, wherein the reactantsubstance supply includes silane as the at least one reactant substanceincluding hydrogen.
 18. The chemical vapor deposition system of claim16, further comprising at least one alternating current power supply sothat the forming of the dielectric layer is a plasma enhanced chemicalvapor deposition process.
 19. The chemical vapor deposition system ofclaim 16, wherein hydrogen is formed as a by-product of the reaction ofthe reactant substances.
 20. A semiconductor device comprising: asubstrate; and layers, the layers including at least one dielectriclayer on the semiconductor wafer, the at least one dielectric layerincluding at least some deuterium.
 21. The semiconductor device of claim20, wherein the at least one dielectric layer includes at least 1% byweight deuterium.
 22. The semiconductor device of claim 20, wherein theat least one dielectric layer is such that the weight of deuterium is atleast 20% of the weight of all hydrogen included in the dielectriclayer.
 23. The semiconductor device of claim 20, wherein the at leastone dielectric layer is a passivation layer.
 24. The semiconductordevice of claim 20, wherein the at least one dielectric layer is aninterlevel dielectric.
 25. The semiconductor device of claim 20, whereinthe at least one dielectric layer is an inter-metallic layer.
 26. Thesemiconductor device of claim 20, further comprising additionaldielectric layers that do not contain deuterium.